This invention relates in general to submersible pumps and in particular to a system for monitoring at the surface the pressure and temperature in the pump motor environment.
The submersible pump installations concerned herein include a large electric motor located in the well. The electric motor receives three-phase power over a power cable from the surface with voltages phase-to-phase being commonly 480 volts or more. The electric motor drives a centrifugal pump to pump well fluid to the surface.
It is important to be continuously aware at the surface of the downhole operating conditions. The pressure of the lubricant in the motor is the same as the well fluid pressure, and provides an indication of whether or not the pump is operating efficiently. Temperature also provides an indication of whether or not the motor is overheating, which might possibly cause early failure. U.S. Pat. No. 3,340,500, C. A. Boyd et al, Sept. 5, 1967 discloses a system for monitoring pressure using the power cables as a linkage between downhole sensors and uphole receiving units. The Boyd et al patent superimposes a DC level on the AC power conductors, with changes in the DC level being proportional to the physical characteristic sensed.
U.S. Pat. No. 3,587,076, James R. Grover, June 22, 1971, discloses means for measuring both temperature and pressure by using steering diodes downhole. The diodes direct positive current through one of the sensors and negative current through the other of the sensors to selectively provide a pressure reading or a temperature reading at the surface. While these two patents disclose successful systems, it is desirable to have a continuous reading of both pressure and temperature at the surface.
Also, some of the downhole pressure measuring components other than the sensors, change resistance with temperature. Unless compensated, this resistance change will cause erroneous readings. U.S. Pat. No. 4,178,579, James R. McGibbeny, Dec. 11, 1979, discloses means for subtracting from a temperature or pressure reading the resistance of the downhole equipment with the sensor disconnected. While this compensates for temperature, it requires a relay downhole which might be subject to failure.